1. Field of the Invention
The present invention relates to DC/DC converter control circuits and DC/DC converter systems with function entering a power saving mode in accordance with an external control signal. More specifically, it relates to DC/DC converter control circuits and DC/DC converter systems used for compact-type electric appliances.
2. Description of Related Art
Recent years, even higher performance and improvement of portability have been demanded for portable type electric appliances such as note-type personal computer, cellular phones, and the like. Accordingly, miniaturization of respective electronic components has been required more and more. Furthermore, improvement of continuous duty time while a battery is driven has also been demanded and respective electronic components have been required to lower current consumption, as well.
Furthermore, the improvements such as above are demanded for not only portable type electric appliances but also non-portable type ones such as personal computers forwarding further higher performance. Demand on improvements of LCD (liquid crystal display) widely used for desk-top-type personal computer is a good example of it. More specifically, a bias power source such as backlight for LCD is forcedly mounted on a narrow space provided around outer periphery of the display to obtain a display space as large as possible in a limited area and therefore, high density mounting on a limited small space provided around outer periphery of LCD is highly demanded for desk-top-type personal computers. Furthermore, on higher demands of environmental protection and resource conservation of recent years, there has been required power saving function to stop operation of backlight function for an LCD, a drive for hard disk motor, and the like with appropriate timing and additionally required power saving operation to lower power consumption at minimum essential with respect to DC/DC converter control circuits and DC/DC converter systems which drive the backlight function and the drive.
In a control circuit 800 for a DC/DC converter system 900 as shown in FIG. 7, switching frequency is determined depending on a resistance value of frequency setting resistance RT connected to an oscillating frequency setting terminal (RT). In the DC/DC converter system 900, there is arranged a control terminal (CTL) as a control terminal for power saving mode. An external control signal CTL to be inputted to this control terminal (CTL) controls a bias circuit BIAS via a control terminal (CTL) for control circuit 800, whereby to start-up a bias current generating circuit 110 constituted by the bias circuit BIAS and a PNP bipolar transistor Q10. Provided that an external control signal CTL is positive logic, the bias current generating circuit 110 is activated in response to an external control signal CTL of high logic level, whereby bias current is supplied to various circuit blocks arranged in the control circuit 800. On the other hand, provided that it is an external control signal CTL of low logic level, the bias current generating circuit 110 is inactivated, whereby supply of bias current to the various circuit blocks in the control circuit 800 is stopped and the DC/DC converter system 900 in active state shifts to power saving mode.
FIG. 8 shows a specific example of a start-up control section constituted by a bias circuit BIAS, and PNP bipolar transistors Q3, Q10. Start-up of a reference voltage circuit VREF is made by this start-up control section. In the next paragraph, a description of the start-up control section will be made provided that an external control signal CTL is positive logic.
An external control signal CTL inputted from a control terminal (CTL) is divided by resistance elements RB1 and RB1 in a meaning of electrical potential, whereby divided voltage Va is obtained and inputted to a base terminal of an NPN bipolar transistor QB1. In case the divided voltage Va is same as or higher than forward voltage VBE with respect to emitter voltage of the NPN bipolar transistor QB1, the NPN bipolar transistor QB1 becomes conductive and there flows bias current I100 determined and obtained by the emitter voltage and the resistance element RB3. The bias current I100 is outputted as bias current I200 through a current mirror circuit constituted by PNP bipolar transistors Q10, and Q3. An emitter terminal of the PNP bipolar transistor Q10 and that of the PNP bipolar transistor Q3 are connected to a diode element DB3 (forward voltage is VBE), and a resistance element RB4 (resistance value is RB4), respectively. The bias current I200 is constant current determined by a value of VBE/RB4. The bias current I200 starts up the reference voltage circuit VREF. Diode elements DB1 and DB2 are arranged in serial in the way of a line between a base terminal of the NPN bipolar transistor QB1 and ground voltage, thereby serving as clamp circuit to avoid over voltage application to the base terminal of the NPN bipolar transistor QB1.
In the DC/DC converter system of FIG. 7, an external control signal CTL inputted from the control terminal (CTL) is inputted to the control circuit 800 through its control terminal (CTL) so that power saving mode operation for the control circuit 800 can be controlled. However,the control circuit 800 must include the control terminal (CTL) as its dedicated terminal to control ON/OFF of power saving mode in the control circuit 800. In this case, provided that the control circuit 800 is housed in a package constituted by semiconductor integrated circuits and the like, this package must include its dedicated control terminal (CTL), which causes unfavorable enlargement of package size.
Regardless of portable or non-portable type, miniaturization has been highly demanded for electronic components nowadays. To meet the demands, there is a fear that the dedicated terminal for the control circuit 800 significantly obstructs downsizing of package size for housing the control circuit 800. Due to the existence of the dedicated terminal, degree of freedom of mounting the DC/DC converter system 900 on a narrow open space provided around outer periphery of LCD is limited, which is problematic in terms higher density mounting.